1. Field of the Invention
The present invention relates generally to visual examination apparatus and, more particularly, to an apparatus for examining the eye of a human being by administering a standardized bleaching flash of light to a subject's eye and measuring the photostress recovery time required for the cones of the macula to recover from the exposure to the bright light.
2. Description of the prior art
The retina is the perceptive structure of the human eye and consists of several layers upon which light falls and makes an impression. For practical purposes the retina can be considered to be divided into layers of nerve cells and their fibers. The deepest layer is made up of rods and cones. This is the part of the retina which is sensitive to light. In the central portion of the retina is a small area called the macula which lies in the visual axis of the eye. The fovea of the macula is the point of greatest sensitivity within the retina, and vision of any considerable accuracy is limited to this small area.
The rods and cones are light-sensitive cells. The cones are concentrated in the central portion of the macula (fovea) and are more diffusely scattered in the periphery. There are no rod cells at the fovea which is the area of distinct vision. In the peripheral portions of the retina, outside the macula, there are both rods and cones, with rods alone at the periphery. The outer segments of the rods and cones contain photopigment which changes with the amount of illumination. The rod cells are concerned with peripheral vision and vision in dim illumination (night vision). The cones are involved vision in bright light, vision requiring high spatial discrimination, and color vision. It has been shown that some cones absorb maximally at 440 mm (blue cones), some at 540 mm (green cones), and some at 560 mm (red cones).
It has been known for some time in the art of visual examination that prolonged decrease in visual acuity occurs in a patient with central serous retinopathy after examination with an opthalmoscope. It is also a part of the prior art that a method or test for central serous retinopathy can be performed by (1) determining the best visual acuity of the eye, (2) exposing the eye to the light of an opthalmoscope for approximately fifteen seconds and (3) measuring the time period required for the vision to return to the pre-test level. This method or test is referred to as the photostress recovery test and the time period required for the vision to return to the pre-test level is called the photostress recovery time. The photostress recovery test has been used to diagnose various maculopathies.
Presently, there is no visual examination apparatus capable of reliably testing the recovery of foveal cone pigments after exposure to a bleaching light. The photostress test as heretofore administered by holding an available bleaching light source relatively close to the subject's eye to expose the eye to the light for an period of time and then measuring by a watch the time required for the subject's visual acuity to return to a predetermined level is inexact at best.
The principle disadvantage in this present method of administering the photostress test is the lack of standardization. The intensity of the dazzling light, the duration of the exposure and the distance between the light source and the eye are all variable. The accuracy of fixation of the dazzle light on the fovea, the possibility of the subject blinking and random eye movements are not controllable. Because of this lack of standardization in the photostress test as presently administered, the method is inconvenient and unreliable and, consequently, infrequently used. It requires a physician or a trained individual to administer the test and cannot be used for serious clinical investigation.